PLENTIFUL ENERGY

The magnitude of the Doppler effect can be very important in such accidents.
The fact that it is small in metal, much smaller than in oxide, is the key to the safe
response. Although both the magnitude of the Doppler effect per degree of
temperature change and the magnitude of the temperature change itself in the fuel
are greater in oxide, the principal difference between metal and oxide is caused by
the much higher temperatures in oxide fuel. The high oxide fuel temperatures in
operation mean that very substantial positive Doppler reactivity comes back in as
the fuel temperature has to decrease many hundreds of degrees due to the feedback
effects. The resulting positive Doppler reactivity slows the decrease in power. In
metallic fuel the centerline temperature is only two hundred degrees C or so above
coolant temperature; in oxide it is in the range of two thousand degrees. The small
Doppler effect in metallic fuel allows the power to drop sharply, as very little
countervailing reactivity comes in. In no channel does the coolant temperature rise
to levels close to boiling. At minimum there is a margin of at least 150 o C from
boiling. After the power surge is over, the reactor stabilizes at a low steady power, a
few percent of normal. There is no damage to the plant. The events are not sensitive
to changes in parameters to any significant degree, except for the coast-down times
of the coolant circulating pumps. If coast downs are extended modestly—as they
were in this example—the increase in cooling capability that results at a critical
time gives a significant increase in the safety margins.
7.6.2 Unprotected Control Rod Run-Out
In this type of accident, the control rod is run out of the reactor completely
through operator error, or by a failure in the control system. No safety rods come in
to shut down the reactor. Reactivity is added at the rate given by the run-out time.
The increase in power heats the coolant bringing in feedback reactivity effects
similar to those in the previous accident type. The end state is similar—power and
sodium temperature stabilized at somewhat higher level, with no damage of any
kind.
7.6.3 Unprotected Loss of Heat Sink
The heat produced for electricity generation is absorbed by the steam generating
system in all power reactors, a substantial fraction producing electricity and the
remainder ―rejected‖ to cooling towers or to large sources of cooling water. This is
just the thermodynamics of electricity production. The important point is that the
steam system is the ―heat sink.‖ It absorbs the heat from the system that the reactor
generates. If this is cut off, trouble starts instantly. With no heat sink, there is
nothing to remove the heat being generated. Temperatures increase immediately if
control and safety rods do not act to halt the fission process and shut the reactor
down.
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